DC rating at starting minutes

If our defenders rating was below 6.5 after some minutes of game even our team didn't conceded any goals is this the symbol of opponents have opportunity to score some goals? Because I always seen this if my dcs rating came down at the starting whenever opponents didn't score any goals then suddenly they score minimum 2 back to back goals???

That's why I mostly sub tactically at half time when there's a deadlock, sub even when he's 6.5, it's shown 6 in the notes.